The iPad has been a remarkable success story. Apple sold 15 million of the original model in the first nine months of the product’s existence, a number that blew away even the most optimistic prognostications. With last year’s introduction of the iPad 2, things kept accelerating. In a little less than two years, Apple has sold roughly 60 million iPads, dominating the market it created.

Maintaining Apple’s lead in tablet devices is the job of the third-generation iPad, a product that doesn’t mess with success. Like the iPad 2 before it, this new iPad is not a re-thinking of the original concept. Instead, Apple has chosen to focus on a few areas of improvement while keeping the overall package the same. Though it’s an approach that can frustrate people who are disappointed by anything that’s not a quantum leap, Apple executes it to perfection and reaps the rewards.

In my review of the iPad 2, I suggested a rule of Apple product evolution I called “Jobs’s Law”—that the latest version of any Apple product is likely to be thinner and lighter than its predecessor. The third-generation iPad breaks that law. It’s actually slightly thicker and slightly heavier than the iPad 2, and in many cases users won’t perceive it to be faster.

But the changes Apple has wrought with this iPad aren’t about making it thinner or lighter or faster, but about making it better. And on nearly every front, the third-generation iPad is markedly better than its predecessor.

In mid-2010 with the release of the iPhone 4, Apple introduced us to a new concept—the “Retina display,” so called because the screen was packed tightly with so many pixels that the dots would be imperceptible to the human eye. At 326 pixels per inch, the iPhone 4 and its successor, the 4S, provide text that looks like it was printed on paper and display photos and videos in high definition.

Far and away the most important feature of the third-generation iPad is that it, too, has a Retina display. Its 9.7-inch screen has a resolution of 2048 by 1536 pixels (a total of four times the pixels in the same space), or 264 pixels per inch. Although that’s a lower pixel density than the iPhone’s Retina display, you tend to hold an iPad further away from your eyes than an iPhone, so the Retina definition still works out.

The result is similar to going from an early iPhone to an iPhone 4—it’s a big leap in quality. Text, video, and photos all benefit. Whether you’re reading a webpage in Safari, a long article in Instapaper, or an even longer work in iBooks, text is razor-sharp. Of course, the display on previous iPads was no slouch. But the moment you pick up a third-generation iPad, you can tell the difference. All the slight jagginess and oddly misshappen characters we take for granted on lower-resolution displays just vanish on the Retina display, and you’re left with the same sort of typographic excellence you’d expect in a printed book.

The effect is even more dramatic with photos and video. Pictures reveal small details that simply weren’t there before. A photo that looks just fine on an iPad 2 looks almost undefinably better on the new iPad. It’s the same image, but all of a sudden, there’s much more information there—small textures and tiny details that were previously omitted.

That’s also true with high-definition video. The third-generation iPad’s screen actually contains more pixels than an HDTV. As a result, the Videos app actually has to blow up 1080p videos slightly in order to display them across the full width of the its screen. (The screen of previous iPads didn’t have enough pixels to show a complete HD picture, so it had to either scale things down or cut off the sides of the frame.)

The videos look great. Watching an HD movie or TV show on the new iPad is like having a home theater in your lap. (Well, assuming you’ve got some good headphones, of course. The iPad’s mono speaker seems to be unchanged from the previous model.)

Buyers of this third-generation iPad will love the Retina display, but the fact is that the iPad 2's screen was also excellent. It may be that there just isn't quite as dramatic a contrast between the two screens as there was between the pre- and post-Retina iPhones two years ago. Maybe I’ve been spoiled by my iPhone’s Retina display, or maybe the iPad 2’s display is really that good.

I found the color temperature on the new iPad to be warmer and more yellow than that on the iPad 2. In isolation, both screens seemed perfectly normal. Only when I placed old and new iPads together did I notice that one is slightly warmer than the other.

As with the transition to Retina displays on the iPhone, app developers will need to step up to take advantage of the higher resolution offered by the new iPad’s display. Surely many (if not most) of them knew this day would come, but it’s quite a job for developers to create new, Retina-sized versions of every graphic in their apps, and it may be some time before all iPad apps are updated. (Non-Retina apps look more or less like they did on previous iPads—but on the new iPad’s Retina display those pixels really stand out.)

Still, it’s not all bad news. Text in most apps will take advantage of the Retina display without modification, even if the graphics don’t. There are odd exceptions, however. When I tested Amazon’s Kindle app with the new iPad, I found that its text was pixelated, not Retina-crisp. Presumably Amazon will fix this in an update. I saw some weird behaviors in a few existing apps, but most of them worked just fine even without being updated. And some work better than you’d think—I tried Comixology’s Comics app and discovered that there’s more resolution to those digital comics than I had realized.

If you’re running an iPhone app on the new iPad, it will display it in high-resolution Retina detail—but in a small compatibility window in the center of the iPad screen. (You can, as always, tap a 2x button to make iPhone apps bigger but more pixelated.)